The present invention relates to a method of and an apparatus for modulation of a beam of light. More particularly, this invention is for a substantially flat reflective surface having selectively deformable portions for providing a diffraction grating.
Designers and inventors have sought to develop a light modulator which can operate alone or together with other modulators. Such modulators should provide high resolution, high operating speeds (kHz frame rates), multiple gray scale levels, e.g., 100 levels or be compatible with the generation of color, a high contrast ratio or modulation depth, have optical flatness, be compatible with VLSI processing techniques, be easy to handle and be relatively low in cost. One such related system is found in U.S. Pat. No. 5,311,360.
According to the teachings of the ""360 patent, a diffraction grating is formed of a multiple mirrored-ribbon structure such as shown in FIG. 1. A pattern of a plurality of deformable ribbon structures 100 are formed in a spaced relationship over a substrate 102. The substrate 102 preferably includes a conductor 101. Both the ribbons and the substrate between the ribbons are coated with a light reflective material 104, such as an aluminum film. The height difference that is designed between the surface of the reflective material 104 on the ribbons 100 and those on the substrate 102 is xcex/2 when the ribbons are in a relaxed, up state. If light at a wavelength xcex impinges on this structure perpendicularly to the surface of the substrate 102, the reflected light from the surface of the ribbons 100 will be in phase with the reflected light from the substrate 102. This is because the light which strikes the substrate travels xcex/2 further than the light striking the ribbons and then returns xcex/2, for a total of one complete wavelength xcex. Thus, the structure appears as a flat mirror when a beam of light having a wavelength of xcex impinges thereon.
By applying appropriate voltages to the ribbons 100 and the conductor 101, the ribbons 100 can be made to bend toward and contact the substrate 102 as shown in FIG. 2. The thickness of the ribbons is designed to be xcex/4. If light at a wavelength xcex impinges on this structure perpendicularly to the surface of the substrate 102, the reflected light from the surface of the ribbons 100 will be completely out of phase with the reflected light from the substrate 102. This will cause interference between the light from the ribbons and light from the substrate and thus, the structure will diffract the light. Because of the diffraction, the reflected light will come from the surface of the structure at an angle "THgr" from perpendicular.
If a wavelength of other than xcex impinges thereon, there will only be partial reflectivity when the ribbons are in the xe2x80x9cupxe2x80x9d0 state, since "THgr" is dependent on the wavelength xcex. Similarly, the light will only be partially diffracted to the angle "THgr" when the ribbons arc in the xe2x80x9cdownxe2x80x9d0 state. Thus, a dark pixel will display some light and a bright pixel will not display all the light if the wavelength of the light is not exactly at xcex. It is very expensive to utilize a light source that has only a single wavelength. Commercially viable light sources typically provide light over a range of wavelengths.
For the above described device to function within desired parameters requires that the heights and thickness of the ribbons and reflecting layers to provide structures are precisely xcex/2 when up and xcex/4 when down. Because of variances in manufacturing processing, the likelihood is small that the relative heights will be precisely xcex/2 when up and xcex/4 when down. Therefore, the expected parameters will be much poorer than theoretically possible.
Another difficulty with the above described structure results from an artifact of the physical construction. In particular, once in the down position, the ribbons tend to adhere to the substrate. Texturing the surface of the substrate aids in overcoming this adhesion. Unfortunately, the textured surface substantially degrades the reflective properties of the surface. This degrades the performance of the device.
The ""360 patent teaches an alternate structure as shown in FIG. 3. According to this conventional structure, a plurality of elongated elements are disposed over a substrate 200. A first plurality of the elongated elements 202 are suspended by their respective ends (not shown) over an air gap 204, as in the embodiment of FIGS. 1 and 2. A second plurality of the elongated elements 206 are mounted to the substrate 200 via a rigid support member 208. The height of the support members 208 is designed to be xcex/4. A reflective material 210 is formed over the surface of all the elongated elements 202 and 206.
In theory, the elongated elements 202 and 206 are designed to be at the same height when at rest. Thus, when all the elongated elements are up and at the same height there will be no diffraction. (In fact there may be some modest amount of diffraction due to the periodic discontinuities of the gaps between elongated elements. However, this period is half the period of the grating so that it diffracts at twice the angle of the desired diffracted light. Because the optics are configured to pick up diffracted light from only the desired angle, this unwanted diffraction is not captured and does not degrade the contrast ratio.)
In order to build a structure such as shown in FIG. 3, a layer must be formed of a first material having a predetermined susceptibility to a known etchant. Portions of that layer are removed through known techniques such as photolithography and etching. A second material is then formed in the voids of the removed material such as by deposition. This second material has a known susceptibility to the etchant which is different than the first material. The layer is formed of the elongated element material. This structure is etched to form ribbons of the elongated elements. Finally, the second material is removed by etching to form the suspended elongated elements 202. A popular use for light modulators of the type described in the ""360 patent is for use as a variable optical attenuator, VOA, for signals in a fiber-optic network.
FIGS. 4A and 4B show how an articulated one-dimensional grating can be used to control the amount of light reflected into an optical fiber. FIG. 4A illustrates a reflective grating 320 in an undeformed state in which an incident light 310 from an optical fiber 305 impinges upon the reflective grating 320. A numerical aperture (NA) of the optical fiber 305 determines an acceptance cone 315 in which the optical fiber 305 accepts light. In its undeformed state, the reflective grating 320 behaves much like a mirror; the incident light 310 is simply reflected back into the optical fiber 305 with no attenuation . FIG. 4B illustrates the reflective grating 320 in a deformed state in which the incident light 310 is diffracted at predominantly predetermined diffraction angles 325. The diffraction angles 325 can be adjusted to be larger than the acceptance cone 315 of the optical fiber 305 thereby allowing attenuation of the incident light 310. By controlling the deformation of the grating, the amount of light reflected back into the fiber can be controlled.
Unfortunately, when arbitrarily polarized light impinges on a linear one-dimensional (1D) grating, each polarization state interacts with the grating differently. Such a scenario is illustrated in FIG. 5 in which an incident light beam 350 impinges upon a 1D grating 360 comprising a series of reflective ribbons placed in parallel. The incident light 350 includes a polarization state P and a polarization state S. Light polarized parallel to the ribbons (polarization state P) interacts with the 1D grating 360 differently than light polarized perpendicular to the ribbons (polarization state S). Polarization states S and P each xe2x80x9cseexe2x80x9d0 different environments at the 1D grating 360. This can lead to Polarization Dependent Losses (PDL) in which one polarization state is attenuated more than the other. These problems become especially acute as the gap between each adjacent ribbon approaches the wavelength of the incident light.
What is needed is a grating system that treats each polarization state equally. Further, a system is desired that substantially eliminates Polarization Dependent Losses. What is also needed is a variable optical attenuator in fiber optic networks that does not suffer from Polarization Dependent Losses.
According to embodiments of the present invention, a light modulator performs variable optical attenuation in fiber optic networks without incurring Polarization Dependent Losses. Preferably, the light modulator is a two-dimensional (2D) MEMS (MicroElectroMechanical System) diffraction grating. The 2D diffraction grating modulates an incident beam of light. A plurality of elements each have a reflective surface with their respective reflective surfaces substantially coplanar. Alternatively, the reflective surfaces of the plurality of elements lie within one or more parallel planes. The elements are supported in relation to one another. Preferably, a planar member includes a plurality of holes arranged in a symmetrical two-dimensional array and configured such that the holes substantially optically extend the elements. In an alternative embodiment, one or more elements substantially optically extends the plurality of holes. The planar member includes a light reflective planar surface that is parallel to the plane of the elements within a functional area of the device. The planar member is supported in relation to the elements. By applying an appropriate biasing voltage to the planar member, the planar member can be moved in a direction normal to the plane of the elements. When the planar member and the plurality of elements are in a first configuration, the 2D diffraction grating reflects the incident beam of light as a plane mirror. When the planar member and the plurality of elements are in a second configuration, the 2D diffraction grating diffracts the incident beam of light. Preferably, the planar member is a membrane circumferentially coupled to a support structure.